Pi-conjugated polymers have been used as semiconductor materials for an organic electronic device such as organic solar cells, organic EL elements, organic thin-film transistors, and organic luminescence sensors. Improvements in the absorption efficiency of solar light are desirable in organic solar cells in particular, and development of polymers capable of absorbing light at long wavelengths (600 nm or more) is important. Examples have been reported in which a copolymer of a donor monomer and an acceptor monomer (hereunder also called a copolymer) is used in a photoelectric conversion element with the aim of increasing the absorption wavelength.
Specifically, Non-Patent Document 1 describes a photoelectric conversion element using a copolymer having an imidothiophene structure and a dithienocyclopentadiene structure. Further, Non-Patent Documents 1, 2 and 3 and Patent Document 1 describe a photoelectric conversion element using a copolymer having an imidothiophene structure and a dithienosilole structure. Moreover, Non-Patent Document 2 describes a photoelectric conversion element using a copolymer having an imidothiophene structure and a dithienogermole structure.